This invention relates to waste water treatment plants and is concerned with the provision of a practical waste water treatment plant which incorporates a blend of structural integrity, mechanical reliability and environmental compatibility. The terms waste water or sewage, as used hereinafter in the specification and claims, is intended to also include nonpolluting sediments and organic-inorganic pollutants such as industrial wastes.
Present sewage treatment plants employ aspects of aerobic and/or anaerobic processes. The term aerobic designates the presence of dissolved oxygen in the treatment process and the term anaerobic designates the absence or deficiency of dissolved oxygen in the treatment process. More particularly, this invention relates to a sewage treatment plant which generally employs an activated sludge process. The activated sludge process commonly includes a primary treatment and a secondary biological treatment of the sewage.
Before undergoing secondary biological treatment, waste water or sewage is commonly subjected to a primary treatment to reduce and remove insoluble and oversize materials. Devices which can be employed in the primary treatment include, either singly or in combination, coarse screens or racks, grit chambers, primary settling tanks, mechanical material reducers and other similar equipment. The purpose of screens and racks is to remove larger particles of floating or suspended material from the waste water. Similarly, grit chambers resemble modified settling basins in which the horizontal flow velocity of the waste water is controlled so that heavier solids, such as sand and metal, are removed while lighter material remains in suspension. Primary settling tanks are used in some activated sludge plants for removing material which settles rapidly from waste water.
The general purpose of the secondary biological treatment is to remove organic material and to clarify and purify the effluent. Generally, the organic material is removed by the aerobic processes of oxidation of organic material to carbon dioxide, water and other derivatives, which is known as respiration, and conversion of the organic material into new material, which is known as synthesis. The activated sludge process may be described as combining organic material, such as sewage, with activated sludge, which contains aerobic bacteria, and oxygen to yield carbon dioxide, water, energy and other products, wherein the energy released during this process allows the synthesis of the new material. The amount of oxygen necessary to oxidize the organic material is primarily dependent upon the biological oxygen demand (BOD) that is required during this process.
Conventional activated sludge plants conmonly employ an aeration chamber, a clarifier and an excess sludge holding tank. Organic material such as raw sewage is mixed with activated sludge, which contains the aerobic bacteria, and the mixture is aerated in the aeration chamber until the organic material is suitably biodegraded which usually occurs within six to eight hours. After aeration, the mixture of organic material and activated sludge, referred to as mixed liquor, is transferred to a clarifier or settling tank where the solids settle out of the solution in the form of a sludge, which contains aerobic bacteria, and the remaining solution, referred to as effluent, may be further treated in separate purifying or chlorine contact tanks. The settled activated sludge is subsequently recycled, mixed with raw sewage and aerated in the aeration chamber. However, because only about 25 to 50 percent of the settled activated sludge is recycled to the aeration chamber, the remaining 75 to 50 percent of the settled sludge must be removed from the system. This factor makes it especially difficult to provide a unitary waste water treatment facility which is efficient, compact, easily serviceable and portable.
Other factors which affect the performance of sludge plants are sludge bulking and septicity of the treatment chambers. Sludge bulking occurs when the activated sludge does not settle well and as it becomes less dense the relative volume of a given mass increases. As the relative volume of the sludge increases, it becomes difficult for the sludge return pumps to keep up with the large volumes of light sludge settling in the tanks. Septicity is often evident on the floors of sedimentation and aeration units where sludge solids accumulate and, when encountered, the septic solids should be removed to prevent the plant from becoming inefficient. To help prevent the occurrence of sludge bulking and septicity, the sludge solids should be properly aerated and controlled. However, in plants which have a large amount of internal reinforcement, complicated piping and tanks which are not streamlined, deleterious effects, such as septicity and sludge bulking, are difficult to control. Accordingly, the present invention relates to treatment tanks having a streamlined design which incorporates little additional internal reinforcement and an uncomplicated pattern of piping.
A relatively recent modification of the activated sludge process is the extended aeration activated sludge process, hereinafter known as the extended aeration process. The extended aeration process, which particularly relates to the present invention when the loading is usually less than 25,000 gallons per day, is characterized by a relatively low loading range, as compared with the conventional activated sludge process, with an objective being to oxidize the biological material produced by synthesis from the removal of BOD. To achieve this objective, the mixed liquor is aerated for an extended period of time, such as for 24 hours, with sufficient oxygen and a deficient amount of organic material. Under this condition, the aerobic bacteria undergo endogenous respiration, wherein the bacteria become self-consuming. Under theoretical conditions, no excess activated sludge will be produced during a treatment cycle and the rate of activated sludge produced will equal the rate of activated sludge consumed. Practically, however, activated sludge is lost from the system as suspended solids in the effluent and as a result of periodic partial wasting of the activated sludge. Various insoluble products produced during the biological degradation process and suspended solids, such as sand and grit, tend to accumulate because the activated solids are recycled and it is necessary, for efficient plant operation, to periodically remove a portion of the sludge, which contains the accumulated insoluble particles and suspended solids. However, after a periodic wasting, the amount of activated sludge present in the system can be built up to desired levels by controlling the operating parameters of the system.
The present invention also relates to a waste water treatment plant which incorporates a blend of economical fabrication and structural integrity. More specifically, the treatment plant of the present invention includes a horizontal elongated first tank, which is divided by partitions into aerating, holding and purifying or chlorine contact chambers, and an upstanding second tank used as a clarifier and uniquely secured to an end wall portion of the first tank. The partitions in the first tank also provide additional longitudinal structural support for the second tank and reinforce the connection between the tanks. An additional feature of the present invention relates to a first tank which may be wider than the second tank at the connection region of the two tanks and, as a result, the first tank includes generally vertical end wall plates which provide reinforcing means for the tank connection in a direction transverse to the longitudinal axis of the first tank. Furthermore, when the longitudinal axes of the tanks are perpendicular and intersect each other a particularly stable structure is formed. Therefore, the combination of a transverse partition, an end wall and a longitudinal partition not only strengthens the first tank against shear, compression and tension forces, torsional stresses and bending moments but also reinforces the connection between the tanks without overdesigning the connection, which may allow the structure to fail in an undesirable mode such as a fracture.
In the preferred embodiment, the first and second tanks are cylindrical. By employing the structural integrity of the circle, the hydrostatic force of the water is transferred to a hoop stress in the circular steel tanks. It is envisioned that the structure of the present invention may be used as either a stationary plant or a portable plant. Stationary plants are usually designed for both stationary loads, such as the weight of the structure and various liquid levels to be contained therein, and dynamic loads, such as would result from equipment operation and flowing liquids. Portable plants should include not only design criteria associated with stationary plants, but also additional design criteria relating to stresses applied to the structure during shipment. Also, foundations for portable treatment plants often fail to provide the same measure of support provided for more permanent structures, thus requiring that portable structures be compact in design and sound in structural integrity.
There have been various attempts to develop portable water treatment plants which incorporate various features of structural integrity, economical fabrication and design, and mechanical reliability. For example, U.S. patents issued to J. Pagnotti (No. 3,152,982), F. Weis (No. 3,161,590), G. Kibbe (No. 3,195,727), M. Nottingham (No. 3,206,032), W. Wagner (No. 3,260,368), E. Simmons (No. 3,371,033), R. Fifer (No. 3,460,677), and A. Thompson (No. 3,767,051) relate to unitary treatment plants. Further, patents issued to A. Marmo (No. 3,684,703) and J. Yang (No. 3,694,353) relate to treatment plants which employ an extended aeration activated sludge treatment process.